Tetrahymena is a ciliated eukaryotic unicellular organism belonging to the regnum of Protozoa and bearing two nuclei, a transcriptionally silent, diploid germline micronucleus (MIC) and a transcriptionally active, polyploid somatic macronucleus (MAC). In 1923, when Nobel Laureate Andre Lwoff succeeded in growing Tetrahymena in pure culture, the basis for exploiting this Alveolate as a model organism was laid. Milestone discoveries made in Tetrahymena are the discovery of dynein motors, telomeres, RNA-mediated catalysis, telomerase and the function of histone acetyltransferases in transcription regulation. Within the last decades molecular biological techniques have been developed to alter Tetrahymena's genome and proteome: DNA transfection methods range from microinjection into the MAC, over electroporation into the MAC to biolistic bombardment of MIC and MAC. Episomal plasmids based on an rDNA-replicon are available, as well as knock-out/-in techniques based on homologous recombination. On protein level heterologous expression of related species has been performed and also endogenous proteins were silenced by a novel antisense-ribosome-technique. The biotechnological potential of Tetrahymena has been proven in numerous publications, demonstrating fast growth, high biomass, fermentation in ordinary bacterial/yeast equipment, up-scalability, existence of cheap and chemical defined media.
So far, only a few promoter active Tetrahymena DNA sequences have been characterised up to a useful extend to work with in molecular biology. These comprise the rather weak and moreover cellcycle dependent histone-1 and beta-tubulin-promoter2 and also heavy-metall-inducible metallothionein-promoters.3,4 Metallothioneins, that are upregulated upon stress, are metal binding proteins playing a role in detoxification of the cell. In Tetrahymena metallothioneins are known, the promoters of which can be induced quite well by the presence of Cadmium, Copper, Zinc—induction becoming less in this order—but barely with other stress factors like peroxides or heat. As heavy metals are toxic, inhibit cell growth and lead—at higher concentrations—to cell death, a different system with less impact on the health of the system and an inducer that can be removed tracelessly would be favourable. Furthermore due to the heavy impact on the environment by heavy metal cations the disposal of contaminated media is time and resource consuming. As the financial effort in commercial bioproduction processes have to be minimal, Metallothionein promoters do not seem to be the inducible system of choice.